1. Field of The Invention
The present invention relates to conveyor belts and, more particularly, to a prestretched conveyor belt having a plurality of layers of oppositely angled cord members that abut each other at the centerline of the belt and an internal stretch limiting means to allow the belt to remain in tension while traversing horizontal curves.
2. Description of the Invention Background
Elongated endless conveyor belts have been developed for traveling through a curvilinear orbit with a generally horizontally extending upper surface for carrying material overlying a generally horizontally extending lower surface. The endless belts themselves are actually composed of one or more conveyor segments that are spliced together in an end-to-end fashion to obtain an elongated conveyor belt in the shape of an endless loop. In general, such belts are stretched upon installation. This pretensioning enables the entire surface of the belt to remain in tension, while allowing the outer edge of the belt to elongate and the inner edge of the belt to shorten, as the belt negotiates a horizontal curve.
Without the incorporation of a means to limit the stretch of the belt due to pretensioning, the forces acting on the belt during use would be sufficient to cause failure in the belt after only very limited use. The incorporation of a stretch limiting means into the length of the belt restricts over-stretching of the belt during use and assures that the predetermined tension is evenly distributed along the full length of the endless loop belt. The stretch limiting means thus provides stability to the belt which allows the entire length of the belt, from edge to edge, to be stretched to and maintained at a predetermined tension. Maintaining a predetermined tension throughout the length of the belt permits the inner and outer edges of the belt in a horizontal curve to experience compression and tension relative to the pretensioned state of the belt, respectively, while traveling through a horizontal curve, but to return to the predetermined tension when no longer traveling through a horizontal curve.
The need for stretching belts to aid in negotiating horizontal curves requires that they be constructed from an elastomeric material such as rubber. Such materials alone have been found to lack the lateral stiffness necessary to allow the belt to resist undue deformation while negotiating horizontal curves. Attempts to develop belts that address this concern have included various combinations of rubberized surfaces containing angled layers of reinforcing bands imbedded therein. These combinations teach that bands are positioned to run across the entire width of the belt, oriented at a given angle of bias with respect to the transverse axis of the belt.
In addition to increasing the lateral stiffness of the belt to aid in resisting undue deformation, the use of angled cords has been found to decrease the tendency of the belt to cut and tear and to add strength to the splices between belt segments. Increasing the angle of the cords increases the strength of the splices between belt segments and thus increases the overall strength of the endless belt. However, increasing the angle of bias of the cords also leads to undesirable sideways forces that work to pull the belt to either side. To maintain these forces at satisfactory levels, it has been necessary to limit the angle of bias of the bands. However, decreasing the angle necessarily weakens the splice strength between each of the connected segments in the endless loop conveyor belt and thus lowers the overall strength of the belt.
Various belt configurations have been attempted in an effort to achieve sufficient splice strength while not inducing unmanageable sideways forces. One such design discloses a belt embodying an internal stretch limiter in conjunction with a lower and an upper layer of reinforcing rods disposed across the entire width of the belt at opposing angles relative to the transverse axis of the conveyor belt. Another belt discloses a reinforced belt comprising a flexible composition such as rubber and having at least two reinforcement layers comprised of steel cords embedded within the flexible composition. The two reinforcement layers are taught to contain cords running uninterrupted across the entire width of the belt, in substantially parallel relation to one another and oriented at a given angle of bias with the transverse axis of the belt. The angle of bias of the cords in one layer is in the opposite sense to the angle of bias of the cords in an adjacent layer. Another embodiment of this belt teaches a similar belt in which two layers of equally angled uninterrupted cords are positioned at the top of the belt and two layers of oppositely angled uninterrupted cords are positioned at the bottom of the belt. Other inventions have taught conveyor belts having a longitudinal reinforcing layer and a number of transverse reinforcing layers constituted by parallel elements spanning the entire width of the belt that are inclined with respect to the median plane of the belt.
Each of these belt designs falls prey to one or more of the above-mentioned problems related to sideways forces and splice strength. Likewise, not all of the references detailed above adequately address the need for the belt to be able to both trough and maintain tension while traversing a horizontal curve. In particular, each of the references above finds it necessary to limit the angle of bias in order to maintain sideways forces at manageable levels. However, as mentioned above, reducing this angle also reduces the overall strength of the belt and, in particular, reduces the resistance of the belt to tearing.
The subject invention is thus directed toward an improved conveyor belt which addresses, among others, the above-discussed needs and provides a conveyor belt that has superior tear resistance and splice strength, a design that negates the build-up of sideways forces during operation due to cord angle and maintains the ability to both trough to provide for retention of material thereon and remain in tension across its entire width while traversing horizontal curves.